The Claus process is widely used by the industry for the production of elemental sulfur. The process is designed to carry out the Claus reaction: ##STR1## The reaction is favored by decreased temperature and by removal of vaporized elemental sulfur.
In the conventional Claus process, the operating conditions of the reactors in which the Claus reaction is carried out are selected to maintain elemental sulfur in the vapor state. Otherwise, the elemental sulfur would deposit on the catalyst and deactivate it. To assure high conversion rates, the reaction is carried out in a plurality of consecutive reactors. Elemental sulfur is condensed and removed from the effluent of a preceding reactor before it is passed to a subsequent reactor. The removal of sulfur permits the reactors to be maintained at progressively reduced temperatures.
Generally, presently desired recovery levels in the range of 97-99% necessitate the use of a modified Claus process which includes a thermal reactor, two catalytic reactors and two low temperature catalytic reactors, such as, for example, cold bed adsorption (CBA) reactors in which elemental sulfur is adsorbed or deposited on the catalyst. The reaction in a CBA reactor is generally carried out at a temperature range, for example, from about 250.degree.-280.degree. F. (121.degree.-138.degree. C.) which results in the condensation of elemental sulfur on the alumina catalyst. The low temperatures in the CBA reactor favor the reaction and the condensation of sulfur removes it from the reaction phase thereby allowing more H.sub.2 S and SO.sub.2 to react. The sulfur condensing on the catalyst deactivates it. Accordingly, a second CBA reactor is provided so that while the first CBA reactor is in the recovery mode, the second reactor is being regenerated and vice versa.
Although acceptable recovery levels can be achieved by the above-described conventional modified Claus process, pressing environmental considerations may soon require significantly improved recovery levels of 99.9% or higher. In order to keep pace with growing environmental concerns, there exists a need to improve this conventional modified Claus process such that optimal recovery levels can be assured.
U.S. patent application Ser. No. 343,661 (now U.S. Pat. No. 4,430,317) by Reed, Petty and Goddin filed on Jan. 28, 1982, a continuation-in-part of application Ser. No. 239,730 filed Mar. 2, 1981, discloses an improved Claus tail gas clean up process for use in a sulfur recovery process. The application issued as U.S. Pat. No. 4,430,317 and is incorporated herein by reference. This tail gas clean up process may be used together with the conventional modified Claus process. However, even when these two processes are used in conjunction, actual recovery will fall significantly short of theoretical recovery because of losses caused by leaking valves. In the conventional modified Claus process, the regeneration gas flow is routed to the reactor on regeneration by introducing a pressure differential in the flow stream at the second condenser. This results in a pressure differential of as much as 5 psi between the reactor on regeneration and that on adsorption. Valves that can give leak-free operation under these conditions are very expensive steam-jacketed globe valves of German manufacture (Gutermuth valves) at critical locations in conventional modified Claus plants.
Further, in the conventional modified Claus process, effluent from the first catalytic reactor is used for regeneration. This effluent comprises about 15% sulfur compounds and 30% water by volume. Regeneration with wet, high-sulfur gas contributes to actual losses.
Thus, there is a long-felt and unsatisfied need for a process and system that will maximize both actual and theoretical sulfur recovery levels by eliminating losses from leaky valves and wet, high-sulfur regeneration gas and overcome the above-mentioned disadvantages of prior art processes.